In this 18-second clip at YouTube,
[The “click me” text overlaid on the clip is an ad.]
a tree branch straddles two power lines, and the branch dies a horrible flameful death. There is weird screaming emanating from the tout-ensemble. It is :eek:, very very :eek:.
What is going on?
It is the sound of water and resins being vaporized at escaping at high rate. Just as wood will do in a less energetic situation such as a camp fire.
That wouldn’t have happened in the first place unless the branch was wet. The screaming sound was probably steam escaping from pores, cracks in the bark, etc. All the while, it’s heating up from the current, and when enough water is finally gone, the heat is enough to cause it to go up in that flash of flame.
Wow! That sound was EXTREMELY upsetting to my cats!
Someone’s been watching too much *Twilight Zone. *Is there a cemetery nearby?
I can’t watch the video from here, but it sounds like you’re describing the kind of squealing caused by gases expanding and escaping through restricted spaces.
I realise that’s already been said, but I wanted to mention a couple of possibly similar phenomena:
Dry ice - when it’s pressed by something hard, it makes a squealing sound as gas escapes across the surface of contact.
Similarly, if a coffee pot or pan is placed wet on a flat hotplate, the resulting steam will often make squeaky noises as it escapes through the narrow gap between the two surfaces in contact.
darn, and here i was hoping it was the sound of the poor plant dying … 
Tell me about the screaming of the trees **aruvqan
**. Do you still hear them at night?
Is that arc at the end–when the branch is no more–an example of a spark gap? Is a spark gap the “first ladder” in a Jocob’s Ladder? 
It looks like they threw the branch up there. There don’t appear to be any trees above the lines.
rolfamao
Sounds like me after Taco Bell
It looks to me like the branch is still there. A little the worse for wear, I’m sure, but that’s one tough branch. I think the sparks only stopped because a fuse blew.
That’s not really what I would call a “spark gap”, but the arcing is the same as what you’d get over a spark gap. You get arcing like that whenever you have two conductors close enough that electricity starts to flow (an arc will jump about 1 cm for every 30,000 volts of electricity). Once you’ve got the electricity flowing, you can pull the conductors away from each other and the arc will continue to flow.
What happens at the end is that the branch heats up and burns. The water is released from the branch, the wood is chemically changed, and the branch no longer conducts electricity all that well. The electricity then finds a better conductor through the air, and you’ve got your arc.
The arc may have extinguished itself at the end. When you have AC, the electricity alternates back and forth 60 times a second (50 in some parts of the world), so every time it switches direction (120 times per second) the electricity flow goes to zero, giving the arc a chance to extinguish. Of course it’s also possible that a protective device on the line detected the fault and shut the line off. If someone threw that branch onto the lines (hard to tell from the video but I don’t see any nearby tree it could have fallen from) they may have caused a bunch of folks to lose power in their homes.
This is a similar arc coming from some high voltage switches being opened.
BTW, a spark gap is generally two conductors placed close enough to each other that when voltage is applied a spark jumps between them, like in an automobile spark plug. A Jacob’s Ladder is a type of spark gap.
This one’s interesting: http://www.youtube.com/watch?v=kVXi_0H_ZzM